Polymeric actuators, electroactive polymer actuators, electromechanical polymeric actuators, artificial muscles, and
other, are usual expressions to name actuators developed during the last 15-20 years based on interactions between the
electric energy and polymer films. The polymeric actuators can be divided into two main fields: electromechanical
actuators working by electrostatic interactions between the polymer and the applied electric fields, and
electrochemomechanical actuators, or reactive actuators, working by an electrochemical reaction driven by the flowing
electric current. The electromechanical actuators can be classified into electrostrictive, piezoelectric, ferroelectric,
electrostatic and electrokinetic. They can include a solvent (wet) or not (dry), or they can include a salt or not. Similitude
and differences related to the rate and position control or to the possibility or not to include sensing abilities are
discussed.

Oxi-reduction processes of conducting polymer are the base of a great number of technological developments in the fields of polymeric actuators (artificial muscles) or smart windows. Hence, the understanding the structural changes that take place in the polymer as a function of its oxidation seems to be crucial for a proper understanding of these complicated systems.
In this sense, a model with atomic detail has been simulated by Molecular Dynamics Simulation, which provides an insight of how the electrical response of the system depends of the structural changes that take place inside the polymer. In this regard, the conducting polymer, water and counterions were modeled with atomic detail with the goal of obtaining an insight of the ring orientation and reorientational relaxation time of the pyrrole rings at different oxidation states of the polymer. In addition, we studied how the above properties are greatly affected by the oxidation state of the polymer and the variation these properties changes from the polypyrrole/water interface to the polypyrrole bulk. Finally, we correlated the reorientational dynamics of pyrrole rings with the oxidation kinetic observed from a macroscopic point of view.

The Molecular Dynamics Simulation technique has been used to describe the behavior of the polypyrrole/water interface at two different oxidation states: neutral (or reduced) and charged (or oxidized) polypyrrole state. The system was modeled by two symmetric and amorphous polymer layers, each one containing 64 polypyrrole chains with 10 monomeric units per chain and 2677 water molecules. When the oxidized polypyrrole was modeled, 128 chloride ions used as counterions to balance the excess of charge of the oxidized polypyrrole.
From the simulated trajectories, several properties with atomic detail have been evaluated such as volume changes during oxidation or reduction process, the atomic and charge distribution profile across the polypyrrole/water interface, and the transitional diffusion coefficient and dehydration of chlorine ions from bulk water to the interior of the polymer matrix. In this sense, a diminution of the hydration and translational diffusion coefficient was obtained for the chloride ions when they penetrated into the polymer matrix.

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